Control systems for various apparatuses rely on a plurality of switches to input data to the apparatus. Switch arrays based on mechanical switches mounted on a surface are well known in this regard. Simulated switch arrays that utilize touch screens are also well known. Mechanical switch panels are difficult to reconfigure, and hence, tend to be customized for each application. This substantially increases the cost and product cycle time of devices that utilize such panels.
Input devices based on touch screens provide a convenient method for inputting commands in a manner that is easily reconfigurable. In such devices, the button pattern is displayed on the screen and the user selects a “button” by touching the screen over the button image. Touch screens, however, are significantly more expensive than conventional switches, and hence, are not always feasible for many applications. In addition, the ambient light intensity can interfere with the display that prevents the user from seeing the simulated buttons. Furthermore, the device cannot distinguish between an accidental light touch and an intended button push, since the screens do not provide a measure of the pressure with which the user has pressed his or her finger on the screen.
Many touch screen designs depend on detecting a change in some electrical parameter such as resistivity or capacitance that varies with the location on the screen at which the user touches the screen. Since the observed changes depend on the shape of the screen, custom screen sizes are limited and can require special programming.
A generic switch panel that significantly reduces these problems is disclosed in co-pending U.S. patent application Ser. No. 10/810,157, which is hereby incorporated by reference. In that system, a switch panel is constructed from a touch plate, image generator, and imaging system. The touch plate includes an optically transparent layer having first and second sides. The optically transparent layer having an index of refraction greater than that of air. A light source generates a light signal that is reflected between the first and second sides of the touch plate within the transparent layer. The imaging system records an image of the first surface of the touch plate. When a user presses a finger on the touch plate, a portion of the internally reflected light is reflected toward the second surface at an angle greater than the critical angle and the light escapes the second surface. The location of the bright spot created on that surface is recorded by the imaging system. A simulated button push is then generated based on the location of the detected bright spot.
The light seen by the camera is the sum of the ambient light that passes through the transparent layer and the light from the light source that is reflected out of the transparent layer. In the embodiments discussed in the above-described patent application, the light source and camera are monochromatic. The background is differentiated from the light source by turning the light source on and off, and then creating a difference image by subtracting the image seen by the camera when the light source is off from that seen by the camera when the light source is on. Unfortunately, in environments having high ambient light, this procedure is not always sufficient.